There are number of fields of human endeavor wherein it is useful, if not necessary, to know precisely the location and orientation of an object within a space. Surgery is one such field in which this information is desirable. Surgical navigation systems are available that enable medical personnel to know, with a high degree of precession, the location and orientation of surgical instrument or implant relative to a surgical site on the patient. Often this information is used in surgical procedures to facilitate the accurate removal and shaping of tissue. In an orthopedic surgical procedure, the information provided by the surgical navigation system ensures that an implant is precisely positioned.
Surgical navigation systems and other position-locating systems use different means to identify the locations and orientations of the objects they track. A number of commercially available surgical navigation systems rely on light tracking to determine the position of the tracked object. Some systems for include trackers that are attached to the objects being tracked. Each tracker emits a number of light beams. Often light is emitted in the infrared wavelengths. A static device, referred to as a localizer, has light sensitive-receivers. Based on the locations from which the individual light beams are received at a localizer, a processor, also part of the system, determines both the position and orientation of the tracker. Based on this information, the position and orientation of the device attached to the tracker is inferentially determined.
Often, at the start of a medical procedure, the position of the patient's body tissue is mapped into a memory integral with the processor. Based on these data and the inferential determination of the tracked object, the surgical navigation system presents an image on a display that indicates the position of the tracked object relative to the body tissue. This allows a surgeon to virtually “view” the position of the object that is otherwise be concealed by overlying tissue.
In an orthopedic surgical procedure, a surgical navigation system is also used to measure the range of motion of the body limb(s) subject to the procedure. These measurement data facilitate the fitting of the implant to the patient to increase the likelihood of successful outcome of the procedure.
Light-based surgical navigation systems work reasonably well for providing object location and orientation data in a surgical setting. Nevertheless, there is a drawback associated with these systems. A light-based navigation system requires a line-of-sight between the light emitting components and the light-sensitive localizer. If the line is broken, the ability of the system to provide object position and location data may be interrupted. Thus, medical personnel using such system must make a concerted effort to keep their own body parts as well as other surgical devices from entering into the space wherein such lines-of-sight may be present.
If the breaking of a line-of-sight results in the interruption of the generation of the object position and location data, it may be necessary stop the procedure until the system can again provide the data. Such delays reduce the overall efficiency of the surgeon performing the procedure. Moreover, such delays can increase the overall length of time it takes to perform the procedure. This is counter to an objective of modern surgical practice, to perform the procedure as quickly as possible. Surgeons work to this goal to reduce the amount of time the patient is held under anesthesia and his/her body is exposed and open to infection.
Recently, there have been efforts to employ electromagnetic field-sensing systems as surgical navigation systems. Generally, this type of navigation system includes one or more transmitters that emit electromagnetic fields. There is a sensor with one or more antenna sensitive to the electromagnetic fields. To provide both position and location information about an object, it is typically necessary to transmit plural fields and monitor the strength of each signal at plural antennae. Some of these transmitters emit electromagnetic fields upon being energized by AC drive signals. Others of these transmitters emit electromagnetic fields upon being energized by DC pulse signals. Based on the strength of the electromagnetic fields measured by the sensor, a processor determines the position and orientation of the sensor relative to the transmitter.
An electromagnetic navigation system does not require a line-of-sight path between the transmitter and sensor. Thus a surgeon could allow his/her arm to enter the space between the system's transmitter and sensor without being concern that such action will result in the interruption of the generation of the object position and orientation-defining data.
Nevertheless, care must be taken when using an electromagnetic navigation system, especially in a surgical setting. This is because metal objects exposed to electromagnetic waves from a first source, in turn, generate their own electromagnetic waves. When ferrous metals, such cold rolled steel, are exposed to magnetic waves, the metal itself becomes magnetized. The metal, in turn, generates its own magnetic fields. This added magnetic field is sensed by the sensor. This added magnetic field thus introduces an error into the magnetic field measurements made by the sensor.
Some metal, such as aluminum, copper, brass and 300 Series stainless steel are non-ferrous. When this type of metal is exposed to a changing magnetic field, a loop current, called an eddy current, develops around the metal. The eddy current, which is changes over time, generates its own magnetic field. This magnetic field, like the magnetic field generated by a ferrous metal object, can introduce an error into the magnetic field measurements made by the sensor.
In surgery, it is often necessary to introduce one or more metal instruments into the surgical in order to accomplish the desired procedure. Many of these instruments have metal parts. For the reasons discussed above, these instruments serve as sources of supplemental magnetic fields that introduce errors into the measurements made by the system sensor. These errors, in turn, can result in the system generating position and orientation information about the tracked object that may not be accurate. In a surgical procedure, and most other procedures in which such navigation is employed, such inaccuracies are wholly unacceptable.
A number of proposed systems sense and/or correct for the errors induced by the extraneous magnetic fields generated in the environment wherein the tracking is performed by electromagnetic field sensing. Some of these systems have transmitters that output AC signals. Some of these systems have transmitters that generate plural magnetic fields to each antenna. Systems wherein the transmitter includes plural parallel-aligned antenna have also been proposed. A disadvantage of many of these systems is that they require their complementary processors to perform numerous calculations in order to generate data representative of the “adjusted”, eddy current-effect free, strength of the sensed magnetic fields.
Other proposed systems include providing the sensor unit with a calibration sensor. These systems thus require the addition of added component to device that it is desirable to keep as compact as possible. Moreover, these systems similarly require their processors to engage in numerous processing steps in order to produce output data representative of adjusted strength of the magnetic field.
Some of the proposed systems monitor the strength of the magnetic fields generated due to the generation of DC pulse currents. Some of these systems measure the magnetic field or the integral of the change in the magnetic field, ∫∂B/∂t, at a time after the magnetic pulse is initially generated. The logic behind waiting this time period to make the measurement is that effects of the eddy currents will have attenuated to a nil level. One disadvantage of these systems is that it delays when, during the signal processing cycle, the magnetic field is measured. This delays when the processor is able to determine object position and orientation. Also, given the relatively long period in which the signal is emitted, these systems can only provide updated sensor position and orientation data at relatively slow frequencies.
Still others of these systems do not actually measure the actual magnetic field, its rate of change or any related integrals. These systems, instead, monitor the profile, the strength, of the magnetic field generated as a consequence of the initial emission of the DC pulse. Based on these measurements, a value representative of the eddy current-free magnetic field is calculated. The logic behind this process is that, since the effect of the eddy current diminishes over time, the initial plot of field strength should, in theory, serve as a basis for calculating the strength of the eddy current free magnetic field. In practice, it has been found that these calculations do not result in the determination of values that accurately represent eddy current-free magnetic field strength. Consequently, the accuracy of the object position and location data produced from these adjusted magnetic field strength data is open to question.